Ionomeric polymers with ionomer membrane in pressure tolerant gas diffusion electrodes

ABSTRACT

Gas diffusion electrodes and gas generating or consuming electrochemical cells utilizing the same are disclosed. The electrode includes an electronically conductive and electrochemically active porous body defining respective gas and electrolyte contacting surfaces, with an ionomeric ionically conductive gas impermeable layer covering the electrolyte contacting surface. The layer includes a layer of a hydrophilic ionic polymer applied directly to the electrolyte contacting surface and a membrane of a hydrophilic ion exchange resin overlying the polymer layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on PCT International application No.PCT/U.S. Pat. No. 88/00621, filed Mar. 2, 1988, which is acontinuation-in-part of copending, commonly assigned application Ser.No. 20,748, filed Mar. 2, 1987 and now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to gas diffusion electrodes and, moreparticularly, this invention relates to gas diffusion electrodes adaptedfor use in electrochemical cells utilizing an aqueous alkalineelectrolyte and consuming or generating a gas via the electrochemicalprocess occurring within the gas diffusion electrode.

2. Description of Related Art

The use of gas diffusion electrodes in fuel cells and metal-airbatteries is well known. Gas diffusion electrodes have also been used inthe electrolysis, either oxidation or reduction, of gaseous reactants.It is also possible to generate gases in such electrodes. In general,gas diffusion electrodes take the form of solid porous (gas and liquidpermeable) bodies formed at least in part of an electronicallyconductive, electrochemically active material, and may include acatalyst. Such electrodes generally define an electrolyte contactingsurface and a gas contacting surface. Electrochemical oxidation andreduction occur at the points in the electrode where the gas to beoxidized or reduced contacts both the electrolyte and the activematerial of the electrode. In the case of gas generation, electrolytecontacts the active material and gas is generated at this interface.

Electrochemical cells utilizing such electrodes generally comprise thegas diffusion electrode, a spaced counter electrode, a liquidelectrolyte (which is generally aqueous) which contacts both the counterelectrode and the gas diffusion electrode, and a gas which contacts thegas diffusion electrode either (1) for reduction or oxidation of the gasor (2) produced via electrolytic generation. Circuit connections aredisposed between the counter and gas diffusion electrodes. Additionally,the counter electrode may also be a gas diffusion electrode. A wellknown example of such a design is the H₂ /O₂ fuel cell.

Electrochemical batteries, for example, the metal-air type, commonlyutilize either an aqueous alkaline or neutral (e.g., saline)electrolyte, while fuel cells may commonly utilize either acidicelectrolytes or alkaline electrolytes. Other types of electrolytes arealso used, depending upon the specific gas which is consumed orgenerated.

The use in electrochemical batteries of an oxygen-containing gas such asair which is reduced at the gas diffusion electrode is well known.However, the gas need not be oxygen-containing nor need it be reduced atthe gas diffusion electrode. For example, hydrogen gas is oxidized insome fuel cells. The present invention is generally applicable to allsuch types of gas diffusion electrodes and cells.

The electronically conductive material in a gas diffusion electrodetypically may be carbon. Additionally, a wide variety of catalysts suchas platinum or transition metal organometallic catalysts (such asporphyrins) are available.

In various applications, it is desirable that either or both the liquidelectrolyte and the gaseous electrode reactant be flowed through thebody of the cell over the electrode surfaces. Flowing electrolyte and/orflowed gaseous reactant are of course accompanied by a pressure dropacross the cell, especially on the electrolyte side. This can be lead toexcess pressures either on the gas-side or the electrolyte-side of theelectrode. Furthermore, it may be desirable in certain circumstances tooperate at an elevated gas pressure with respect to the electrolytepressure. One example of such a situation would be one in which theperformance is increased by pressurizing the gaseous reactant. Inbattery and fuel cell applications, it is desirable to obtain as high acell voltage as possible at any given current density. One means ofaccomplishing this is to utilize a relatively high gas pressure or flowrate.

The use of a porous (e.g. typically 30-60% porosity) gas diffusionelectrode, however, poses difficult flow management problems. When gaspressure exceeds liquid electrolyte pressure by a sufficient amount,"blow-through" of gas through the electrode into the liquid electrolyteresults. In conventional gas diffusion electrodes, this so-called"blow-through pressure" is usually much lower than is desirable fortolerance of substantial differential pressures between the gas andliquid sides of the cell.

For example, while it may be desirable to operate a cell at a gas vs.liquid differential pressure of up to 10 psi or more, typical aircathodes exhibit a gas blow-through pressure of less than about 0.25psi. If the differential pressure exceeds the blow-through pressure,pumping of gas into the liquid electrolyte may result. (Typicalblow-through pressures range from 0-1 psi, and are determined primarilyby interfacial tension and pore size distribution.)

Conversely, if the liquid electrolyte pressure is higher than the gaspressure and the differential pressure exceeds the liquid bleed-throughpressure, liquid may be pumped into the gas side of the cell, which mayresult in liquid in the gas manifold, with consequent pumping problemsand a decrease in cell performance and useful cell life due to floodingof the active layer of the electrode.

In gas-generating cells, it is customary for the gas to be generated onthe front face (electrolyteside) of the electrode. The gas is thusgenerated as bubbles in the electrolyte, which can lead to removal ofelectrolyte from the cell and increased ohmic losses. Generation of gasin a gas diffusion electrode is more desirable because the gas can exitthe cell directly through the back of the electrode. Operation in thismode would require a certain amount of pressure tolerance. Even higherpressure tolerance would be required if the gas is generated in apressurized state.

If the differential pressure between the gas and liquid sides of anelectrochemical cell using a porous gas diffusion electrode is to bemaintained at a low level, impractical pressure management problemsresult, especially in view of the fact that pressure levels vary frompoint to point on each side of the electrode.

The problems described are not readily amendable to correction by theuse of a gas barrier material between the gas and electrolyte sides ofthe electrode, since such barriers tend to block the flow ofelectrolytic ions through the electrode and also strongly contribute tovoltage losses or do not allow operation at a sufficiently high currentdensity for the desired application. It is desirable to maintain thepotential across the electrode at as positive a level as possible whilemaintaining as high a current density as possible. For example, it maybe desired to operate a cell at a current density of up to as high as500 mA/cm², typically at 100 mA/cm², while minimizing the voltage lossacross the electrode. A voltage loss of less than 0.05 volts ispreferred, with voltage losses of up to 0.25 volts being generallyacceptable.

One approach to solving these problems is disclosed in Juda and IlanU.S. Pat. No. 4,614,575 (Sept. 30, 1986), which involves the use ofnonionic polymeric hydrogel as a layer applied by painting onto theelectrolyte side of the gas diffusion electrode. The maximum pressuretolerance disclosed by the Juda, et al. patent is less than or equal to40 inches of water (1.44 psi or 10.0 kPa), which is significantly lessthan that possible with the present invention.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome one or more of theproblems described above.

According to the present invention, an ionomeric, ionically conductive,substantially gas impermeable layer is disposed over substantially theentire electrolyte contacting surface of a gas diffusion electrodeadapted for use in a gas generating or consuming electrochemical cellutilizing a liquid electrolyte. The layer comprises a layer of ahydrophilic ionic polymer covered by a membrane of a hydrophilic ionexchange resin.

The invention also comprehends an electrochemical cell comprising thecoated gas diffusion electrode spaced from a counter electrode and incontact with a liquid electrolyte. A gas to be oxidized, reduced orgenerated is in contact with the gas side of the electrode, and circuitconnections are disposed between the counter and gas diffusionelectrodes.

The electrode and cell of the invention are capable of operating at veryhigh gas vs. electrolyte differential pressures at high currentdensities without significant voltage loss.

Other objects and advantages of the invention will be apparent to thoseskilled in the art from a review of the following detailed descriptiontaken in conjunction with the drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transverse sectional view of one embodiment of anelectrochemical cell in which the invention may be utilized;

FIG. 2 is a schematic sectional view of a typical gas diffusionelectrode with which the invention may be utilized;

FIG. 3 is a sectional view of an electrode holder useful in testing gasdiffusion electrodes;

FIG. 4 is a schematic exploded perspective view of an electrode assemblyadapted for use with the electrode holder of FIG. 3:

FIG. 5 is a schematic transverse sectional view of an electrode as usedin FIGS. 3 and 4;

FIG. 6 is a polarization curve exhibited by an electrode with a cationexchange membrane for comparison with FIGS. 7 and 8;

FIG. 7 is a series of polarization curves exhibited by an embodiment ofan electrode made according to the invention; and

FIG. 8 is a polarization curve exhibited by another embodiment of theelectrode of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a typical embodiment of an electrochemical batteryutilizing a gas diffusion electrode. This particular cell is an aqueousalkaline lithium-air cell. It is to be understood that the presentinvention is not limited to use in electrochemical batteries, nor tocells in which gas is consumed. Rather, the invention finds wideapplicability in cells in which gas is either consumed or produced, viaeither reduction or oxidation, in which any of various electrolytes areused, etc.

The cell of FIG. 1 is described in detail in U.S. Pat. No. 4,528,249(Jul. 9, 1985) the disclosure of which is incorporated by reference.

In FIG. 1, an electrochemical cell, generally designated 10, includes ananode 11, a gas consuming cathode 12, and a metal screen 13 interposedbetween the anode 11 and cathode 12 within an outer housing 14. In theembodiment of FIG. 1, the screen 13 is in electrical contact with thecathode 12, and is in mechanical (but not electrical) contact with theanode 11.

In the exemplary embodiment, the anode 11 comprises a lithium anode,which may comprise elemental lithium metal or lithium alloyed withalloying material such as small amount of aluminum.

The screen 13 is not in electrical contact with the anode 11, due to thepresence of an insulating, porous lithium hydroxide (LiOH) film which isformed on the anode surface by contact thereof with humid air, and iswell known in the art. It is to be noted, however, that this particularfeature is peculiar to the aqueous lithium-air cell. In other types ofmetal-air batteries and fuel cells, either an electrically insulatingporous separator layer or a simple electrolyte gas would be used. Itshould also be noted that the screen 13 is necessary to help restrainthe gas diffusion electrode 12 against the gas pressure.

The cathode 12 is in this case an air cathode through which atmosphericair flows. Those skilled in the art, however, will recognize that such acathode may operate with any oxygen-containing gas.

One surface 15 of the cathode 12 is exposed to ambient atmosphere (or asource of another oxygen-containing gas) in a chamber 16 of the housing14, and the opposite surface 17 of the cathode 12 is contacted by theliquid electrolyte 18 which is flowed through a second chamber 19 in thehousing 14 as by a suitable pump 20. In the illustrated embodiment, theelectrolyte is provided from a reservoir 21 for suitable delivery whenneeded.

In FIG. 1, the anode 11 and cathode 12 each terminate in a respectiveterminal 26 or 28, and are connected to a load 30 through suitablecircuit connections 32.

Typically, the cathode 12 comprises a structure formed of a suitableporous hydrophobic material, such as polytetrafluoroethylene (PTFE),mixed with carbon black, both pure and catalyst-containing. A preferredform of the cathode 12 is described below in connection with FIG. 2.

The screen 13 illustratively may comprise a woven metal wire screenformed of suitable non-corroding metal, which in the case of alkalineelectrolyte may be nickel or silver plated nickel. If desired, thescreen 13 may serve as a current collector if connected to the terminal28.

In the embodiment of FIG. 1, liquid electrolyte, in this case an aqueousalkaline electrolyte such as aqueous lithium hydroxide, is flowedthrough the chamber 19 by means of the pump 20. As such, there is apressure drop across the chamber 19 in the direction of flow.

Further, air is flowed through the chamber 16 by means not shown, andthere is a small pressure drop across the chamber 16 in the direction offlow by virtue thereof. However, those skilled in the art will recognizethat the pressure drop across the gas chamber 16 is small in comparisonto that in the electrolyte chamber 19.

As set forth above, FIG. 1 is intended to be exemplary only, as theinvention is applicable to any of a variety of types of gas diffusionelectrodes and electrochemical cells.

FIG. 2 is a schematic depiction of the structure of a preferredembodiment of the cathode 12. As shown in FIG. 2, the electrode 12 isformed essentially of a two or three component laminate defining the gascontacting surface 15 and the opposed electrolyte contacting surface 17.An electronically conductive porous gas carrier layer 40 defines the gascontacting surface 15 and typically is a mixture of a hydrophobicmaterial such as porous PTFE (e.g. Teflon brand PTFE) with a carbonblack such as Shawinigan black (Chevron Chemical Co., Olefins andDerivatives Div., Houston, Tex.). A so-called "active layer" 42comprises a layer 44 which comprises a mixture of carbon black, orcatalyst supported on carbon black, and PTFE. An optional layer 46 ofcatalyst is disposed on the layer 44 at an interface 50. As shown in theschematic of FIG. 2, layers 44 and 46 appear to be discrete layers, butin practice may define a single layer or two layers, since the catalystis generally adsorbed onto the surface of the material of layer 44. Insome cases, the materials of the three layers 40, 44 and 46 may beintermixed in a single layer.

The entire structure of the electrode 12 of FIG. 2 is porous, generallyexhibiting a porosity of 30-60%.

A typical catalyst forming the layer 44 is heat-treated cobalttetramethoxypheny porphyrin (CoTMPP) on a carbon black such as VulcanXC-72 (Cabot Corp., Billerica, Mass.). The heat treatment is typicallydone at 400-1000° C. in inert gas. The structure of CoTMPP is shownbelow: ##STR1##

This material is a currently preferred catalytic material. Othercatalysts include platinum, MnO₂ and transition metal macrocycles otherthan CoTMPP.

The function of the layer 40 is to allow ready transmission of gas tothe active layer 44. Its hydrophobicity also acts to repel liquidelectrolyte which exists in the active layer 44 in order to avoidleakage of the liquid electrolyte into the gas side of the cell. It alsoprovides electronic conductivity.

The requisite consumption or generation of gas takes place in the activelayer 44 where gas and liquid meet in the presence of the activematerial and optional catalyst, as is well known in the art.

FIG. 3 illustrates an electrode holder useful in measuringcharacteristics of gas consuming or generating electrodes. The electrodeholder, generally designated 60, comprises a solid body 62 of anonconductive material defining a gas inlet passage 64 communicatingwith a cell gas chamber 66 which in turn communicates with a gas outletpassage 68. (A typical material of construction for the body 62 is 3M'sKel-F brand choloro fluorocarbon polymer.) An annular electrode seat 70is defined in the body 62 in order to position an electrode assembly(not shown in FIG. 3) which includes a gas diffusion electrode,generally designated 72, adjacent the cell chamber 66. A conductive(e.g. platinum) wire 74 contacts the seat 70 and extends therefromthrough the outlet passage 68. A threaded plug 76 of the same materialas the body 62 retains an electrode assembly 80 (shown in FIG. 4) inplace in the body 62.

FIG. 4 illustrates the electrode assembly, generally designated 80,which includes the gas diffusion electrode 72 of FIG. 3. The electrode72 is shown in schematic form in FIG. 4 and formed as a cylindrical diskdefining gas and electrolyte contacting surfaces 82 and 84 respectively.These surfaces are analogous to surfaces 15 and 17 of FIG. 1. An annularconductive metal (e.g. platinum) ring 86 is disposed on the gas surface82 between the gas surface 82 and an annular rubber gasket 88. A similarrubber gasket 90 is disposed on the electrolyte side of the electrode 72between the electrolyte contacting surface 84 and an annular ring 92 ofthe same material as the body 62.

When the assembly 80 is in place in the seat 70 of the electrode holder60, the ring 86 is in electrical contact with the wire 74 and acts as acurrent collector.

The electrode 72 as shown in FIGS. 3 and 4 is schematic and thesefigures do not illustrate certain components such as the hydrophobicbacking layer and asscociated screens. FIG. 5 illustrates an explodedsectional schematic view of a typical embodiment of the diffusionelectrode 72. A silver plated nickel screen 100 is adjacent to and incontact with an electronically conductive hydrophobic backing layer 102,typically of Teflon brand PTFE plus carbon black, which defines thesurface 82. An active layer 104, which may include a catalyst on carbonblack, is adjacent to the layer 102 and defines the surface 84. Ahydrophilic layer 106 is applied to the surface 84 and is in contactwith a steel reinforcement screen 108. The layer 106 is described indetail below.

When constructed, the screen 100 is not in physical or electricalcontact with the ring 86 and thus merely acts as a physical restraint.The gas inlet passage 64 and gas outlet passage 68 are connected withgas flow regulating means (not shown) which regulate the flow of gasthrough the passages 64 and 68 and the cell chamber 66, and thus the gaspressure in the chamber 66.

Those skilled in the art will recognize that the screens 100 and 108 maybe embedded in the layers 102 or 106, respectively, and that the layers102 and 104 may form a single homogenous layer if desired.

When the electrode 72 is in place in the assembly 80 in the electrodeholder 60, a central circular segment of each of the electrode surfaces82 and 84 is exposed to gas and electrolyte sources, respectively. Theelectrode holder body 62 is positioned in a test cell such that theelectrode surface 84 is exposed to a flowing or non-flowing (e.g.stirred) electrolyte. The remainder of this cell and associatedtemperature control means, etc. are omitted for clarity.

For operation at elevated gas/electrolyte differential pressures, thesteel screen 108 acts as a reinforcement to prevent physical rupture ofthe electrode 72. Flow-through of gas from the cell chamber 66 throughthe electrode 72 into the electrolyte side of the cell is prevented bythe layer 106 as described below.

The layer 106 directly overlies the active layer surface 84 of theelectrode. The layer 106 comprises a hydrophilic ionic polymer coveredby a membrane of a hydrophilic ion exchange resin which is substantiallyimpermeable to the gross passage of gas. The material of the layer 106is ionically conductive to hydroxide (OH) ions as well as water. It isalso possible for bulk electrolyte to slowly diffuse through themembrane. The electrode 72 may be effectively wetted through the layer106, while the layer 106 is virtually impermeable to gas flow.

The ionic polymer is applied directly to the electrolyte contactingsurface as by application of an aqueous or organic liquid solution ofthe polymer. Preferably, for use with alkaline electrolytes, the ionicpolymer is a cationic polymer (anion exchange), and the membranecomprises an anion exchange resin (i.e. the membrane includes cationicand/or nonionic groups in the polymer chain or pendant therefrom). Foruse with acidic electrolytes, the ionic polymer is preferably anionic(cation exchange) and is used in conjunction with a cationic exchangeresin (i.e. a resin including anionic and/or nonionic groups). Apreferred ionic polymer for use with alkaline electrolytes is poly(dimethyl diallyl ammonium chloride), (PDMDAAC, 15% solids in water,Polysciences, Warrington, Pa.). The combination of the ionic polymer andthe membrane does not allow the macroscopic flow of gases, but doescontain and transport ions, thus creating a pressure tolerant electrode.

The combination of the ionic polymer layer and the ion exchange resinmembrane is synergistic, in that the ionic polymer tends to keep themembrane from "bowing" away from the active layer under high gaspressures, and the membrane prevents the ionic polymer from washing offin electrolytes in which the polymer may be soluble.

A wide variety of ion exchange resin membrane materials are useful, andinclude those wherein a polymer backbone is grafted, as with aquaternized vinyl benzyl amine. Such backbones include fluorinatedpolymers such as polytetrafluoroethylene (PTFE). Other useful membranesinclude quaternized ammonium polymers such as tetraalkyl ammoniumpolymers and perfluorinated polymers such as perfluorosulfonic acidpolymers. A preferred type of perfluorinated polymer is sold by DuPontunder the trademark NAFION.

Perfluorinated resins have the dual advantages of high oxygen solubilityand extemely high chemical stability.

In addition to NAFION polymer, a preferred ionic polymer for use inacidic electrolytes is poly (styrene sulfonic acid) (PSSA, 30% solids inwater, Polysciences).

In either case, the ionic polymer migrates into the body of theelectrode and into the ion exchange resin membrane. It is believed thatthe presence of the ionic polymer may "open up" the membrane such thatthe degree and extent of crosslinking of the membrane is effectivelydecreased. This should not significantly alter the operation of themembrane, however.

The presence of the ionic polymer may also effectively occlude anyphysical pinholes which may be present in the ion exchange membrane,especially on contact with aqueous electrolytes in which such polymersare swellable.

By the use of the invention, it is possible to achieve extremely highgas/electrolyte differential pressures and to achieve high voltages,i.e. minimize the loss of voltage due to the barrier.

EXAMPLES

The following specific examples are intended to illustrate the practiceof the invention and should not be considered to be limiting in any way.

The following generalized experimental procedure was used to prepare thegas diffusing electrode.

Cobalt tetramethoxyphenyl porphyrin (CoTMPP) was adsorbed on VulcanXC-72 carbon (Cabot) by agitating a suspension of the latter in asolution of 10⁻⁴ M CoTMPP in acetone for at least 24 hours. The amountof the adsorbed macrocycle was calculated by spectrophotometricallydetermining its loss from the filtered solution. The solidcatalyst/carbon was airdried and then heat-treated to 450° C. in ahorizontal tube furnace under continuous flow of purified argon.

Porous gas-fed electrodes were fabricated as follows: dilute ˜2 mg/mL)Teflon T30 B aqueous suspension (DuPont) was slowly added to an aqueoussuspension of the catalyst/carbon while the latter was ultrasonicallyagitated. The mixed suspension was then filtered out with a lμm poresize polycarbonate filter membrane. The paste was worked with a spatulauntil slightly rubbery. The paste was shaped into a 1.75 cm diameterdisk in a stainless steel die using hand pressure. This disk was thenapplied to another disk, ˜0.5 mm thick, of Teflon-carbon blackhydrophobic porous sheet material (Eltech Systems Corp., FairportHarbor, Ohio), which contained a silver plated Ni mesh. This dual layerdisk was pressed at 380 kg cm⁻² at room temperature and thenheat-treated at 290° C. for 2 hours in flowing helium.

The gas-fed electrode was placed in a Teflon- Kel-F electrode holder asshown in FIG. 3. The gas (O₂ or air) pressure was applied to theback-side (hydrophobic layer) of the electrode and was monitored at theoutlet. A needle valve at the outlet was used to regulate the gaspressure.

The O₂ reduction measurements for the gas-fed electrodes were donegalvanostatically in a concentrated alkaline electrolyte (0.5 M LiOH in2:1 v/v 50% NaOH and 45% KOH) at 80° C. with a research potentiostat(Stonehart Associates, Model BC1200). This potentiostat is equipped withpositive feedback IR drop compensation and correction circuits. The IRdrop correction adjustment is made while monitoring the potential on anoscilloscope, with the current repetitively interrupted for 0.1 ms every1.1 ms. This procedure corrects for any IR drop that is external to theelectrode itself. Nickel foil was used as the counter electrode and aHg/HgO, OH⁻ reference electrode was used. The polarization curves wererecorded under steady-state conditions.

For purposes of comparison, an air cathode was prepared by applying aNAFION polymer membrane (about 0.018 millimeter thick) and a stainlesssteel screen over the active layer of an electrode as shown in FIGS. 2and 4-5. No ionic polymer layer was present. This electrode was testedin oxygen reduction at one atmosphere (no pressure differential betweengas and electrolyte sides) in a 50 wt. percent sodium hydroxide aqueouselectrolyte at 80°-83° C. FIG. 6 shows the polarization curve. Theperformance is poor, as expected, because the hydroxide ion has very lowmobility in NAFION polymer, which is a cation exchange membrane.

In comparison, another electrode was prepared by coating the electrodeactive surface with pDMDAAC and then by covering the ionic polymer layerwith a causticresistant anion exchange membrane (AR 108-401) fromIonics, Inc., Watertown, Mass. This electrode was tested at adifferential pressure of 5 psi with both air and oxygen using anelectrolyte as described above at 80° C. The polarization curves areshown in FIG. 7. The data represented by the curves were obtained withboth increasing and decreasing current densities. The potentialexhibited at 100 mA/cm² is excellent and is only ˜15mV more negativethan that obtained for an electrode tested without the polymer andmembrane.

Another air cathode assembly was prepared first by coating with a thinlayer (˜1.25 mg/cm², dry weight) of pDMDAAC and then pressing (˜380 kgcm⁻²) on a disk of tetraalkylammonium polymer membrane material (Ionics,Inc.) over the active layer. This modified electrode can withstand agas-side overpressure of ˜75.0 kPa without gas blow-through. Thisoverpressure tolerance is ˜20-fold higher than is expected for a typicalunmodified air cathode. The O₂ reduction polarization curve is shown inFIG. 8.

Operation at high current densities (e.g. up to about 1 A/cm² or more)is possible according to the invention.

Examination of the polarization curves presented above shows thataccording to the invention, very great increases in current (i.e. incurrent density) are available with only minor increases in thepotential driving force over a wide range of current densities. (TheTafel plots display relatively low slopes, ˜45 mV/decade.)

The foregoing detailed description is given for clearness ofunderstanding, and no unnecessary limitations are to be inferredtherefrom, as modifications within the scope of the invention will beobvious to those skilled in the art.

We claim:
 1. A gas diffusion electrode adapted for use in a gasgenerating or consuming electrochemical cell utilizing a liquidelectrolyte, said electrode comprising an electronically conductive andelectrochemically active porous body defining respective gas andelectrolyte contacting surfaces and an ionomeric ionically conductivegas impermeable layer covering substantially the entire said electrolytecontacting surface, said layer comprising a layer of a hydrophilic ionicpolymer applied as a liquid solution directly to said entire electrolytecontacting surface and a membrane of a hydrophilic ion exchange resindirectly overlying said polymer layer.
 2. The electrode of claim 1wherein said resin comprises a polymer backbone grafted with quaternizedvinyl benzene amine.
 3. The electrode of claim 2 wherein said polymerbackbone comprises a fluorinated polymer.
 4. The electrode of claim 3wherein said polymer backbone comprises polytetrafluoroethylene.
 5. Theelectrode of claim 1 wherein said resin comprises a quaternized ammoniumpolymer
 6. The electrode of claim 5 wherein said resin comprises atetraalkylammonium polymer.
 7. The electrode of claim 1 wherein saidionic polymer is a cationic polymer and said membrane comprises an anionexchange resin.
 8. The electrode of claim 7 wherein said polymercomprises poly(dimetnyl diallyl ammonium chloride).
 9. The electrode ofclaim 8 wherein said membrane comprises a perfluorinated polymer. 10.The electrode of claim 9 wherein said perfluorinated polymer comprises aperfluorosulfonic acid polymer.
 11. The electrode of claim 1 whereinsaid polymer is an anionic polymer and said membrane comprises a cationexchange resin.
 12. The electrode of claim 11 wherein said polymercomprise poly (styrene sulfonic acid).
 13. The electrode of claim 1wherein said porous body comprises a laminate of a porous hydrophobiclayer defining said gas contacting surface, and a porous active layerdefining said electrolyte contacting surface, said active layercomprising an electrochemically active material.
 14. The electrode ofclaim 13 wherein said electrochemically active material comprisescarbon.
 15. The electrode of claim 13 wherein a catalyst is adsorbed onsaid active material.
 16. The electrode of claim 15 wherein saidcatalyst is cobalt tetramethoxyphenyl porphyrin.
 17. An electrochemicalcell, comprising:(a) a gas diffusion electrode comprising anelectronically conductive and electrochemically active porous bodydefining respective gas and electrolyte contacting surfaces, and anionomeric ionically conductive gas impermeable layer coveringsubstantially the entire said electrolyte contacting surface, said layercomprising a layer of a hydrophilic ionic polymer applied as a liquidsolution directly to said entire electrolyte contacting surface and amembrane of a hydrophilic ion exchange resin directly overlying saidpolymer layer; (b) a counter electrode spaced from said gas diffusionelectrode; (c) a liquid electrolyte in contact with said counterelectrode and with said membrane on said electrolyte contacting surfaceof said gas diffusion electrode; (d) either a gas to be consumed viaoxidation or reduction or gas formed via electrolytic generation incontact with said gas contacting surface of said gas diffusionelectrode; and, (e) circuit connections between said gas diffusionelectrode and said counter electrode.
 18. The cell of claim 17 whereinsaid gas is an oxygen containing gas.
 19. The cell of claim 18 whereinsaid gas is air.
 20. The cell of claim 17 wherein at least one of saidliquid electrolyte and said gas are flowed through said cell.
 21. Thecell of claim 17 wherein said resin comprises a polymer backbone graftedwith quaternized vinyl benzene amine.
 22. The cell of claim 21 whereinsaid polymer backbone comprises a fluorinated polymer.
 23. The cell ofclaim 22 wherein said polymer backbone comprisespolytetrafluoroethylene.
 24. The cell of claim 17 wherein said resincomprises a quaternized ammonium polymer.
 25. The cell of claim 24wherein said resin comprises a tetraalkylammonium polymer.
 26. The cellof claim 17 wherein said ionic polymer is a cationic polymer and saidmembrane comprises an anion exchange resin.
 27. The cell of claim 26wherein said polymer comprises poly (dimethyl diallyl ammoniumchloride).
 28. The cell of claim 27 wherein said membrane comprises aperfluorinated polymer.
 29. The cell of claim 28 wherein saidperfluorinated polymer comprises a perfluorosulfonic acid polymer. 30.The cell of claim 17 wherein said polymer is an anionic polymer and saidmembrane comprises a cation exchange resin.
 31. The cell of claim 30wherein said polyner comprises poly (styrene sulfonic acid).
 32. Thecell of claim 17 wherein said porous body comprises a laminate of aporous hydrophobic layer defining said gas contacting surface, andporous active layer defining said electrolyte contacting surface, saidactive layer comprising an electrochemically active material.
 33. Thecell of claim 32 wherein said electrochemically active materialcomprises carbon.
 34. The cell of claim 32 wherein a catalyst isadsorbed on said active material.
 35. The cell of claim 34 wherein saidcatalyst is cobalt tetramethoxyphenyl porphyrin.